Thermal and electromagnetic switch opening mechanism



Jan. 12, 1943.: E TAYLOR 2,308,401

THERMAL AND ELECTROMAGNETIC SWITCH OPENING MECHANISM Filed April 27, 193

w-A. 711851K 51....

Patented Jan. 12, 1943 THERMAL AND ELECTROMAGNETIC SWITCH OPENINGMECHANISM Elmer H. Taylor, Newburyport, Mass., assignor to TheChase-Shawmut Company, Newburyport,

Mass., a corporation of Massachusetts Application A pril 27, 1939,Serialv No. 270,306

Claims.

'I'his invention relates to electric switches and to current responsivetrippingfmeans therefor, and especially to tripping means having athermally actuated element responsive after a delayed period of ltime toeffect the opening of the switchA on moderate overloads and toindependently acting electromagnetic means to effect the instant openingof the switch upon high overloads or short circuits.

'I'here is a type of'switch, especially adapted for motor control,employing a latch member or the equivalent to maintain the co-actingswitch members or contacts releasably in closed position wherein abimetallic strip responsive to the current passing through the contactsis utilized to maintain the latch releasably in switch closed position,the bimetallic strip normally support- .ing the latch against thepressure exerted thereon as by the engaged contacts, the strip beingheated` by the current passing through the switch to warp away fromholding engagement with the latch to permit the contacts to separate.Usually the bimetal strip is so arranged that the latch exerts pressureon the strip in the direction of its length and the strip acts as astrut. The endwise pressure of the latch thereon tends to cause thestrip to buckle and hence the strip has to be made sufllciently heavy toprevent the buckling. When the strip is internally heated, that is tosay, when the strip carries the switch current and is heated by reasonof its resistance to the ilow of such current, the buckling pressure ofthe latch on the strip is of no particular consequence since the striphas to be thick in order to make its resistance low enough to givereasonably satisfactory perfomance. With such an internally heated stripit has been proposed to flex the strip electro-magnetically to obtainthe release the latch instantly upon heavy current.

In my copending application Serial No. 172,412, filed November 2. 1937,I have shown that if the bimetal strip is heated'externally, as by acurrent traversed resistor or heating element disposed in heat impartingrelation to the bimetal strip, the strip could be made thinner and thusthe electro-magnetic means could be made smaller and also that a betterperformance of the switch under all current conditions could beobtained. With an externally heated bimetal strip the buckling pressure.of the latch on the strip becomes important and limits thethinness ofthe K strip and the operating characteristics of the switch that aredetermined by the thickness of the strip. If the tendency of the stripto buckle under the latch engaging pressure were reduced or eliminated astill better performance of the switch under overloads could be obtainedand the electromagnetic means could be made still smaller, which latteris desirable not only for reasons of economy but also because thetripping means rordinarily has to be located in a very confined space,especially in this type of switch.

Hence an object of the present invention is a 4 construction whereby thelatch is held releasably in switch closed position by a member, as astrut member, that receives the entire latch pressure and can be made tohave any desired characteristics and wherein the bimetal strip is freedfrom the latch pressure and can be of such structural dimensions as aredesired to obtain the desired circuit opening characteristics of theswitch upon overload, the only limiting feature of the bimetal stripbeing that it rmust have sufiicient power to overcome the frictionalengagement between the strut member and the latch so as to eiiectdisengagement of the two under the desired conditions of time delay andoverload. The bimetal strip thus can be made thinner than with theconstruction set forth in my above copending application and theelectromagnetic means associated therewith can be made correspondinglysmaller.

yA further object of the invention is generally to improve upon electricswitches and circuit breakers and combined thermal and magnetic openingmeans therefor.

Fig. l is a sectional elevation through an electric switch or circuitbreaker embodying the present invention.

Fig. 2 is a perspective detail of the strut of Fig. 1.

Fig. 3 is a perspective detail of the strut fulcrum of Fig. 1.

Fig. 4 is a section along line 4-4 of Fig. 1.

The switch herein illustrated as embodying the present invention isconstructed generally as i1- lustrated in my copending applicationSerial No. 233,006, led October 3, 1938.

The switch comprises a supporting block i0 of insulating material thathouses and carries the various switch elements. The stationary contacti2 of the switch is connected to a terminal strut Il to which a circuitconductor can be connected by the screw I6. The movable switch contacti8 is carried by a movable switch member or strip 20 having a resilientS bend 22 intermediate its ends and a depending end part 2| clamped to adepending ear 26 by suitable means as the rivet 2B, the ear dependingfrom a horizontal bracket plate 30 fixed to the supporting block i0. v

Said plate 38 has a pair of upstanding arms 32 which carry between thema pivot 34. A switch operating member 38, which operates as a latch, aswill later become apparent, has a slot 38 through which said pin isloosely extended, said operating member having a cam projection 40 whichbears upon the movable switch member 20 and a horizontally extendedlatch projection 42 the end of which bears releasably against the topend face of a strut member 44.

The operating member is moved vertically about the strut member as apivot by an actuating member 46 pivoted on the pin 34 and having amanually engageable operating knob or projection 48. The actuatingvmember has a cam surface 50 which engages a cooperating cam surface 52of the operating member to effect vertical pivotal movements of theoperating member about the strut member in response to swinging of theactuating member in opposite directions to effect the manual opening andclosing of the switch. The arrangement is such that when the strutmember 44 is moved out from under the latch projection 42 the inherentresiliency of the movable switch member causes the operating member tobe rotated in a clockwise direction to effect the automatic opening ofthe switch,

The construction and operation of the switch mechanism as thus describedwill not be explained in further detail as it is unnecessary and themechanism herein illustrated is only one form of mechanism that can beused in connection with the present invention.

In accordance with the present invention said strut member 44 can beformed of a relatively rigid and inflexible strip sufficientlyinflexible to resist the downward pressure of the latch 42 thereonwithout buckling or if the strip does buckle somewhat under the latchpressure such buckling is of no importance providing, of course, thatthe buckling is not sufficiently great to move the parts associatedtherewith out of operative relation with each other. 4

The important feature of the strut member with respect to the presentinvention is that it should support the latch member against itspressure and be capable of movement out from under the latch by thethermal and electromagnetic tripping means, hereinafter to be explained,to permit the latch to move into switch open position.

In order that the thermal and magnetic tripping means shall have aslittle work as possible to do, and hence need have mainly the powernecessary to overcome the frictional engagement between the latch andthe strut, the strut member is made rigid and the lower end thereof ispivotally supported on a fulcrum offering but slight frictionalresistance to the short movement of the strut. To this end the bracketplate 30 on opposite sides of the depending ear 28 is provided withforwardly extending fulcrum ears 54 which have aligned V-shaped fulcrumgrooves 58 in their upper faces. The lower end part of the strut 44 isprovided with opposed spaced depending legs 58 which have V-shaped loweredge parts 80 located in the grooves 58. The arrangement thus gives aknife edged fulcrum support for the strut which permits it to be movedeasily out from under the latch.

The thermal tripping means comprises a straight bimetal strip 82composed of two metals of dissimilar coefficients of expansion securedtogether back to back. Said strip is disposed in relatively closeconfronting parallel relation with the strut member and is secured to.the bracket ear 28 by the aforesaid rivet 28, being maintained inspaced relation with the strut member by an interposed washer 84. Aplate 88 on said rivet is disposed between the legs Il of the strutmember to prevent undue edgewise movement of the strut member on itsfulcrum support.

A cylindrical armature 18 is secured to the upper end part of the strutand extends laterally therefrom on the side opposite the latch 42. Thearmature is provided with an annular groove 12 therein in which theupper end part of the bimetal strip 82 is located loosely. The armature.thus forms a connection between the bimetal strip and the strut memberby which the strip on warping toward the right can move the strut outfrom under the latch and when cooled oi! and re- V turning tostraightened or unwarped position can restore the strut member to itsnormal latch holding position. The resetting movement of the latch canalso move the strut member toward the right, the bimetal strip servingas a spring to restore the strut member into its normal position underthe latch. Preferably the width of the groove l2 is slightly greaterthan the thickness of the bimetal strip.

'I'he bimetal strip is heated by a, preferably selfsupporting externalheating element 14 consisting of a conductor of suitable electricalresistance bent into grid form and disposed in sumciently closeproximity to the front face of the strip ,to impart heat thereto. Oneend of the conductor is connected to the rivet 2l and thus is in serieswith the separable switch members i2 and I8. I'he other end of theheating conductor is connected by means of a bolt 1I with one end of asolenoid 80. Said solenoid, as here shown, consists of a few turns, twoturns being sufficient for some switches, of a low resistance conductor.The other end of the conductor is connected by a screw l2 to a terminalplate I4 to which a circuit terminal may be secured by suitable means asthe screw 82. l

As thus arranged a moderate overload current traversing the heatingelement 14 is arranged to heat the bimetal strip and ultimately after asuitable period of time delay cause the strip to warp sumciently toremove the strut member 44 out from under the latch whereupon the switchmechanism effects downward displacement of the latch member and theseparation of the switch contacts. For moderate overload currents thesolenoid is designed to be ineective so .that it does not exert anysubstantial pull on the strut member. When, however, the switch istraversed by heavy overloads, say upwards of 400 or 500 percent of fullload current, the magnetic effect of the current traversing the solenoidis sufficient to act instantly to move the strut member out from holdingengagement with the latch, thereby to effect instant tripping or openingof the switch upon the occurrence of such heavy current.

With this construction the bimetal strip can be made thinner thanheretofore. Thus the operating characteristics of the bimetal strip arenot restricted by necessary strength requirements heretofore imposedupon the strip and the strip may be given any reasonable characteristicsdesired. For instance. the time delay of operation of the switch uponmoderate currents may be made much less than heretofore and a muchfaster operating switch on moderate overloads can be obtained with thisconstruction while at the same time getting the desirable instantoperation upon higher overloads. By being able to make the strip thinthe electromagnet can be made smaller and cheaper since it is notrequired to flex a stiff bimetal strip.

I claim:

1. Ina switch tripping device, a. switch operating member normallybiased for movement in a direction to effect an operation of the switch,a swingable strut positioned and arranged normally to engage and holdsaid switch operating member from such movement, and electromagneticmeans operative to move said strut out of holding relation, a warpableblmetal strip having a connection with said strut and saidelectromagnetic means to maintain said strut yieldably in the aforesaidholding relation, said bimetal strip constituting also a spring torestore said strut to said holding relation when it has been movedelectromagnetically away therefrom, said bimetal strip warping whenheated and moving said strut away from said holding relation, anelectric heater disposed adjacent said bimetal strip effective to heatand warp it under predetermined conditions, and supporting means holdingthe aforesaid parts in the aforesaid relation.

2. In a switch tripping device, a switch operating member biased formovement in a predetermined direction, a holding strut releasablyengaged with v'said switch operating member and supporting it againstthe bias thereon, and a warpable bimetal strip in close side bysldeparallei relation with said strut arranged when warped to disengagesaid strut from said switch operating member, electromagnetic meansacting independently of said bimetai strip upon said strut in adirection to move it toward disengaging position. and supporting meansholding the aforesaid parts in the aforesaid relation.

3, In a switch tripping devicel a switch operating member normallybiased for movement in a direction to enect an operation of the switch,a swingable strut normally arranged to engage and hold said switchoperating member from said movement, a bimetal strip having a connectionwith said strut to maintain it yieldably in the aforesaid holdingrelation and to restore said strut to said holding relation when it ismoved temporarily away therefrom and to warp when heated and to movesaid strut away from said holding relation, an electric heater disposedadjacent said bimetal strip effective to heat and warp it underpredetermined conditions, and electromagnetic means operative tc warpsaid bimetal strip mechanically to move said strut out of holdingrelation, and supporting means holding the aforesaid parts in theaforesaid relation.

4. In a switch tripping device, the combination as in claim 1 in whichthe said electromagnetic vmeans has an armature connected to said strut,

and the said connection of the bimetal strip to the strut and theelectromagnetic means is through the said armature.

5. In a, switch tripping device, a switch operating normally biased formovement in a direction to effect an operation of the switch. holdingmechanism positioned and arranged normally to engage and hold saidswitch operating member from such movement comprising a pair of swingingplates connected to swing together, one plate being a strut arrangednormally to engage and hold said switch operating member from theaforesaid movement and the other plate being of bimetal warpable whenheated to swing the strut away from its aforesaid holding position andto release saidswitch operating member, electrical means for heatingsaid bim'etal plate, electromagneticemeans for swinging said holdingmechanism, and supporting. means holding the aforesaid parts in theaforesaid relation.

suma n. TAYLOR.

Certificate of Correction Patent No. 2,308,401 January 12, 1943 ELMER H.TAYLOR It is hereby eertied that error appears in the printedspecification of the above numbered patent requiring correction asfollows:

Page 3, second column, line 26, before the Word normally insert member;

and that the said Letters Patent should be read as corrected above, sothat the sam may conform to the record of the ease in the Patent Office.

Signed and sealed this 5th day of December, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certiiicate of Correction Patent No. 2,308,401 January 12, 1943 ELMER H.TAYLOR It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows:

Page 3, second column, line 26, before the Word normally insert member;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the ease in the Patent Ofee.Signed and sealed this 5th day of December, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oommzssz'onar of Patents.

